The invention relates to a method for fixing images in plastics, an adhesive layer, and a fixed image.
The quality of printing methods, in particular digital printing, has drastically improved in recent years. It is possible to produce high-quality photorealistic images, and also to quickly and easily provide other illustrations.
In particular for digital printing, in the future conventional, commercially used printing techniques will find increasing use and also gain tremendous importance in these sectors.
A difficulty heretofore for use of printing techniques in the commercial sector (for example, the design, telecommunications, automobile manufacturing, shipbuilding and aircraft construction, and packaging materials industries, etc.) is the direct transfer of an image to a plastic surface and fixing the image. This problem may be solved in a fairly satisfactory manner using current techniques, but presently there is no known method that fixes images in plastics in such a way that the plastics are thermoplastically deformable to a great extent or even elastically stretchable. Thus, currently there is also no known method that is able to satisfactorily fix images in elastic plastic polymers.
In addition, there are no known methods in which the information in an image is divided over several levels in the plastic, resulting in three-dimensionality. By use of such a method, production of three-dimensional images or veneers, for example, or also applications in model construction would be possible.
However, a method which allows plastics to be imprinted with images at the very beginning of a production line, and only then to further process same (deep-drawing, thermoforming, stretch blow molding, blow molding, etc.), would offer a number of advantages. On the one hand, such a process allows completely new design possibilities for products. Images may be deformed or stretched in three dimensions, and the image may also be easily adapted to complex shapes. A good design is vital for a product. On the other hand, the production costs are significantly reduced by use of the described method. In this manner it is often possible to avoid costly injection molding processes, and also to economically realize lower quantities. Further advantages of this method are the ease of recycling the products, since no composite materials are produced. In addition, as a rule the adhesive layers may be imaged with inks/pigments containing water, and pigments containing solvent are not necessary.
The image transferred to a plastic should have the following properties, in particular for further processing in commercial sectors:                The transfer should be inexpensive and relatively simple        The image should be resistant to abrasion        The image should behave as a part of the plastic itself during elastic stretching or in plastic deformation processes, and be able to completely follow the motion dynamics of the plastic; it should not tear when stretched, and should not experience damage under stress        In the transfer of an image to a plastic, the absolute minimum number of additional layers of other plastics or materials should be necessary, and these should subsequently remain on the image in order to produce bodies that are relatively homogeneous with respect to materials.        
For transferring digital images, for example, to the surface of solid plastics a number of processes have been developed, which may be roughly divided into three groups:
1) In the manufacture of a composite system (DE 102004049592 A1, DE 103 27 453 A1, EP 1647399 A2) ink jet print is first transferred to a system of various films, and the film composite is adapted to the desired shape and then back-molded with plastic.
2) In other processes the ink jet print is directly applied to a plastic surface. There are various methods for this purpose, having the common feature that high temperatures and/or high pressure or assistance from other agents are necessary. These processes are distinguished in that either an entire film imprinted with digital printing is placed on the plastic, or that only a specific layer of a printed film is transferred to the plastic. In many cases it is necessary to apply a further protective layer to protect the digital image from mechanical influences such as abrasion, for example (EP 1464512 A2, EP 0858913 A1, DE 60018808 T2, DE 69620782 T2, EP 1188570 A2, DE 69820120 T2, DE 69805507 T2, and others of Class B41M 5/035). According to the method described in WO 2004/037556 A1 the digital image is printed on a layer situated on a temporary substrate. At the same time, the surface of the plastic workpiece to be imprinted is modified so that it is adhesive, and the digital image may be held by means of adhesion forces when the temporary substrate having the printed layer is pressed onto the plastic to be imprinted. The layer having the digital image is detached from the temporary substrate and remains on the pretreated plastic surface. The temporary substrate may be removed from the workpiece. This method as well usually requires a further protective layer.3) A third possibility lies in the use of inks which initiate a reaction on the workpiece to be imprinted, resulting in fixing of the inks; sublimation inks have a plastic component as the binder. The digital print is first imprinted on a siliconized carrier layer, and then with the use of pressure and fairly high temperature is fused onto the plastic in a transfer press, and the plastic component of the ink melts and bonds with the plastic of the workpiece to be imprinted. The intermediate carrier may then be removed (DE 20017119 U1, WO 03/082595 A1). Substrate-reactive digital printing represents a similar process U.S. Pat. No. 6,402,331). In this case functional groups of certain components in the ink react with functional groups of the workpiece surface, but this process is less suitable for transfer to plastic. A further variant is the use of inks containing a solvent as component which dissolves the surface lacquering of the surface to be imprinted, resulting in fixing of the pigments (EP 1080933 A2).
For transferring images and brand names to elastic objects, there are methods in which the surface of the objects to be imprinted is first pretreated and the surface is cleaned using various degreasing solvents. The surface is then further treated to prepare it for the imminent printing. A screen printing process is generally used for this purpose (DE 19829242 A1, U.S. Pat. No. 3,617,326, and others). An image imprint on a stretched object is also possible (GB 2077661 A, and others). Photorealistic digital printing on a low-ductility material coated with latex is described in WO 96/21701. In this method the inks contain latex components. However, in this patent it is noted that the image is not particularly resistant when heated or when cleaning agents are used, which once again points out the basic problem of digital printing on elastic workpieces.
Transfer of printed images to various levels within a molding made of plastic has not been described heretofore. The superimposition of composite systems would create a nonhomogeneous object, resulting in optical defects. Other known methods for applying digital images would likewise impair the homogeneity of the plastic due to the use of adhesive layers composed of another material (different plastic layers converted by sublimation).